Schlumberger Technology Corporation, the assignee of the present invention, has pioneered the use of Modular Formation Dynamics Testers (MDTs) and other down hole tools. The Modular Formation Dynamics Tester is one of several very useful instruments for obtaining formation fluid samples. The MDT tool is suspended by a wire line and then lowered into the borehole of the well. The instrument is secured to the walls of the borehole and samples of the formation fluid are extracted. Such a tool is illustrated in U.S. Pat. No. 4,860,581, issued to Zimmerman et al on Aug. 29, 1989.
Fluid sampling tools comprise a pumpout module that can be used to draw fluids from the formation, circulate them through the instrument for analysis, and then expel these fluids to the borehole. The MDT can also retain samples of formation fluids in sampling bottles, which are then transported to the surface. The samples are transferred at the surface from the sampling bottles to transportation bottles. The formation fluid samples are then sent to pressure-volume-temperature laboratories (PVT labs) for analysis of their composition and their physical properties. Conventional PVT labs provide a broad range of measurements and services.
It is essential to know the bubble point of crude oil, because when the borehole pressure drops below the bubble point pressure during production, gas bubbles form in the porous rock reservoir. This dramatically decreases the oil phase relative permeability. Knowledge of the bubble point is also useful in determining the composition of the hydrocarbon mixture in the reservoir.
The best current practice of measuring bubble point is to bring a sample of fluid to the surface to be sent to a laboratory. There, the sample is placed in a cylinder, the volume of which is increased by a piston. Pressure is monitored by a gauge. The bubble point is normally considered to be the pressure at which a break (knee) appears in the pressure versus volume (P-V) curve.
However, this technique has several disadvantages. It is time consuming to bring a fluid sample to the surface, transfer it to the (possibly distant) laboratory, and await the result. Further limitations of this technique are: (1) only a few samples (typically six or fewer) can be transported to the surface on each tool run; (2) samples are altered by pressure and/or temperature changes when they are brought to the surface; (3) sample composition can change as a result of imperfect transfer from sampling bottle to transportation bottle, and to laboratory apparatus; (4) typically, a delay of several weeks occurs between the time of fluid sampling and the receipt of the laboratory report; (5) it is not known whether the sample and data are valid until long after the opportunity to take further samples passes; (6) high pressure, toxic, explosive samples must be transported, handled by wellsite and laboratory personnel, and disposed of, creating numerous potential health, safety and environmental problems.
The break in the aforementioned P-V curve is unreliable for determining the bubble point. A more reliable method is to observe bubble formation in the cylinder by use of a sight glass. In this manner, bubbles may be detected visually. They may also be measured by the transmission of near infrared light, since the bubble point is associated with attenuation of the light beam.
A number of down hole measurement techniques have been proposed for making a bubble point measurement within a down hole tool. These methods are described in U.S. Pat. Nos. 5,329,811; 5,473,939; 5,587,525; 5,622,223; and 5,635,631.
As described in the above-mentioned patents, fluid is isolated in the flow line, and then a pump (the same one used to extract fluid from the formation) is used to expand the volume. A pressure gauge is used to monitor the P-V curve.
Several problems exist with these prior art methods of determining bubble point. First, the measurement is very time consuming. At each stage of the expansion, it is necessary to allow bubbles to nucleate.
In U.S. Pat. No. 5,635,631, a gas is formed slowly, "relative to the amount of time taken to expand the sample." A full bubble point determination can require over an hour. Identifying a single pressure, following the maximum expansion, as the bubble point pressure, is clearly inaccurate, since it assumes that the compressibility of the hydrocarbon below the bubble point pressure is negligible. This assumption is erroneous, and can lead to substantial errors in bubble point pressure determination.
To detect phase changes of complex hydrocarbon mixtures, it is necessary to nucleate bubbles or drops of the new phase and to detect these bubbles. In standard laboratory apparatus, and in prior art down hole tools, the bubbles or drops are formed at arbitrary locations in the fluid volume, and then detected by pressure-volume measurements, or by detecting bubbles at another site (e.g., in the beam between a source and detector of light). Both of these methods are characterized by a delay between the arrival at the thermodynamic phase line and the initiation of phase change, and then a delay between the phase change and its detection. The methods and tools of this invention solve both problems.
In a related prior art publication [SPE 30610 (1995) Michaels (Western)] a technique is described in which the volume is increased as the pressure is monitored. Special significance is attached to the pressure at which the P-V curve departs from linearity. The authors cautiously declined to call this pressure the bubble point. This criterion may aid in collecting a sample for surface analysis, but it is not helpful in planning reservoir operations. This pressure may underestimate the bubble point, if the appearance of bubbles is delayed by retarded nucleation. Thus, maintaining the production pressure at this level during oil production may lead to formation of gas in the formation, and thus reduced productivity.
The present invention addresses a method of providing a down hole method of making rapid, accurate measurements of bubble point using a down hole tool, such as an MDT tool.
The dew point is the most important thermodynamic parameter associated with gas condensate reservoirs. Gas condensate reservoirs produce gas at high pressure. As the pressure drops, liquid is formed. When this happens in the pore space of the rock, the permeability to gas flow is greatly reduced, with accompanying loss of production. Therefore, it is important to maintain the pressure of gas condensate reservoirs above the dew point for as long as possible.
Sensors have been developed to measure the dew point of ordinary humid air. A cooled plate provides a definite location for the nucleation of liquid drops. The plate is part of a mass-sensitive sensor, such as an acoustic surface wave resonator, which detects the first presence of the liquid. H. Ziegler and K. Rolf, "Quartz Sensor for Automatic Dew-Point Hygrometry", Sensors and Actuators, Vol. 11, pp. 37-44 (1987).
Devices of this kind will often fail when used to measure the dew point of gas condensates under down hole conditions. This is so, because mixtures of hydrocarbons found in reservoirs can have unusual phase diagrams. As the pressure is reduced, the first condensation of liquid can occur at either the hottest or the coldest point accessible to the mixture. Amyx, Bass, Whiting, "Petroleum Reservoir Engineering", McGraw-Hill, 1960, pp. 220-229.